Powering a passive sub with an old receiver?

I've seen mention of doing this here and I never thought about it until my brother said he was upgrading his receiver and doesn't have the budget for a new powered sub. I'm not quite sure how you would accomplish this. The idea is quite simple, but just not sure of the logistics. Help?

Generally speaking, one can power a passive sub with an old receiver. It sounds like the new receiver will have a line level sub output. One can feed this into a source input of the old receiver (but not the phono input). Then connect the speaker terminals of the old receiver to the passive sub's speaker terminals. The old receiver's volume should be set to a fixed level so that the new receiver's volume will control the output of all speakers. One issue is whether the passive sub can be driven with the power available in the old receiver. Different subs have different power requirements.